Video subsystems for driving video displays typically employ video digital-to-analog converters (DACs). The video DAC can generate a variety of discrete current levels which are sent to the video display via a transmission line. Within a color graphics system, colors are created by "mixing" the three primary colors--red, green and blue (RGB).
Color images are typically stored in color maps by allocating twenty-four (24) bits/pixel (i.e., 8 bits for red, 8 bits for green and 8 bits for blue). Each of these 8 bit codes range from 0 to 255. If every bit is used, this 8:256 decoding scheme allows for a "photo-quality" color system. Video DACs convert the 8 bits of digital information into an analog signal, such as an analog current.
An 8-bit CMOS video DAC for generating either red or blue is typically fabricated by using 276 switchable current cells in the following proportion: (a) 255 cells for video information; and (b) 21 cells for the blanking period. A green video DAC typically adds 110 switchable current cells for a synchronization (sync) signal. In addition, control cells are used to actuate the current cells. The sync signal is usually placed within the green channel, although sync in either the red or blue channel is possible.
Typically, the process of switching the current cells on and off during normal operation creates transient noise on the chip which can couple to the power supply. This noise occurs when current cells are switched because the power supply current changes in step amounts and the inductive nature of the lead frame pins and bond wires reacts to these changes by causing the supply voltage to "bounce." One common approach to address this problem involves putting a "dummy" load resistor on the chip which roughly approximates the resistive component of the off-chip video load. The output of the current cells is then steered between the off-chip video load (when the cell is `on`) and the on-chip dummy load resistor (when the cell is `off`). This results in a reduction in the electrical noise associated with the switching of the DAC output current levels, particularly in the power supply, because the supply current drawn by the DAC remains essentially constant and the supply voltage does not bounce. However, the current running through the on-chip dummy load resistor substantially increases the power dissipation in the DAC, giving rise to thermal and power supply current issues.